The invention involves a commutator with copper segments embedded in plastic, which at the very least engage a receptacle in a reinforcing ring, arranged coaxially with the axis of rotation of the commutator, which comprises a metal ring as well as an insulating ring assembled with the metal ring. The invention further involves a process for the manufacture of such a commutator, having a body comprising copper segments, with at least one receptacle for a reinforcing ring fabricated from a metal ring and an insulating ring, the reinforcing ring introduced into this receptacle and the commutator subsequently cast with plastic.
Numerous implementations of commutators reinforced with fiberglass reinforcing rings are well-known. Despite the considerable advantages of these commutators--for example, fiberglass rings exhibit favorable expansion characteristics and are readily prestressed or reinforced, and furthermore, fiberglass rings can be slipped directly onto copper armature retainers, because the reinforcing rings are also electrical insulators--they still have a disadvantage compared with commutators reinforced with steel rings. This disadvantage manifests itself when these commutators are used in motors at high heat loads or for long operating times under high temperature conditions. It is also possible that any sort of defect can result in a thermal overload. With all thermal overloads, a local softening of the insulating or fiberglass rings can occur if low-cost resins are used. A consequence of this is that the commutator segments can be distorted beyond their tolerance limits, whereby the lifetime of such commutators can be considerably decreased.
Commutators have therefore already been proposed, in which the reinforcing ring consists of at least one metal ring of essentially rectangular cross section, which engages an insulating ring of essentially rectangular cross section. By way of example, one such commutator is well known from German Patent Publication No. DE-OS 4302759. This publication discloses a commutator for an electric motor with fan-shaped copper lamina arranged on its circumference, which is anchored by means of internal lands engaging undercuts in an insulating carrier made of a plastic molding compound. At least one reinforcing ring including a metallic tension ring is thereby enclosed within the carrier, wherein the internal lands engage projections in the region of the undercuts and at the very least produce an insulating intermediate layer on its inner side with respect to the projections. Moreover, the intermediate layer consists of a support ring, closely fitted to the tension ring, and made of a material which is both an insulating material and compression resistant at elevated operating temperatures.
Because a press fit is provided between the tension and support rings, whereby the tension ring and the support ring form a reinforcing ring as a rigid and solid unit, both rings must be fabricated to high-precision dimensions and consequently very close manufacturing tolerances prior to their assembly, so that a constant compressive force between the two rings, as well as a corresponding clearance precision within the undercuts, can always be assured. An additional result is that in the case where the support ring is made of glass, pre-stressing to enable the press fit is only attainable at very high manufacturing costs. Furthermore, it is possible that in applications using a fiberglass ring, the latter can soften at high temperatures if inexpensive, non-heat-resistant resins are used, which at the very least could lead to damaging this reinforcing ring comprising a fiberglass ring and a metal ring. Additionally, the insulating ring or glass ring arranged between the hub and the metal ring can no longer be pre-stressed when installed within the commutator.
A commutator of the above-described type is well known from International Publication Nos. WO95/22184 and WO95/22185. These publications deal with the manufacture of a commutator reinforcing ring assembled from a metal ring and a fiberglass ring. A metal ring of rectangular cross section is thereby pressed endwise into a fiberglass ring of nearly rectangular cross section; in this way, the fiberglass ring is deformed in such a way that a projecting area results, which is displaced on the metal ring and is adjacent to a radial outer surface of the metal ring, whereby a centering or flanged part results. Moreover, the generic state of the art is represented by FIGS. 3 and 6 of WO95/22184 or FIGS. 3 and 7 of WO95/22185. Importantly, there is a clearance between the metal ring and the armature retainer of the copper segments which is filled with plastic. Apart from this, the state of the art is represented by German Patent Publication No. DE-43027159-A, wherein a first part of the radial outer surface of the armature retainer presses firmly against the metal ring of the reinforcing ring across an intermediate layer of high-temperature, compression-resistant plastic.
Underlying the present invention is the problem of specifying a commutator of the type described above, which exhibits an even greater torsional strength at high operating temperatures as well as at high rotational velocities, by technically straightforward means, being at the same time easily manufactured and also still able to take advantage of the expansion characteristics of the insulation ring. Further underlying the invention is the problem of specifying a process of the type described above, which makes possible the manufacture of a commutator with further improved torsional strength at high temperatures and rotational velocities, and which together greatly simplifies its manufacturing process.